Drive shafts are used primarily as a means for transmitting power from a power source, i.e., an engine or transmission, to a driven component, i.e., a tail rotor of a helicopter. The power transmitted is principally a function of the rotational velocity of the shaft, typically designated in revolutions per minute (RPM), and the applied torque or twisting moment.
In designing a drive system, many factors must be weighed in order to provide the optimum design configuration. While it may be desirable to utilize a single drive shaft, one end being attached to the power source and the other end being attached to the driven component, in most instances this would not be feasible for two main reasons. Firstly, the driven component may not lie along the same axis as the power source requiring the drive system to transmit the power along two distinct axes, e.g., first aft, then up.
Secondly, the maximum speed of the drive shaft is limited by the critical shaft speed, above which instability occurs. The critical speed of a shaft is based, in part, on the length between supports, the physical and geometric properties of the shaft, and any loads, e.g., vibratory loads, that are applied to or supported by the shaft. The critical speed is defined as the natural frequency (point of instability) of the shaft.
It is primarily due to these two factors that a typical drive system consists of a series of segmented drive shafts, coaxially disposed, each supported at or near its ends and joined together through a coupling device. The coupling device is designed to transmit torque from an input drive shaft segment to an output drive shaft segment. FIG. 1a depicts a typical helicopter drive system with several drive shaft segments. As can be seen, the driven component (tail rotor) is located aft and above the power source (transmission), requiring the power to be transmitted along two distinct axes through a complicated gear system as shown in FIG. 1b. In a helicopter configuration, where the drive shaft speed can exceed 1800 RPM, torque can be as much as 27000 inch-pounds, and vibratory loads are extremely common, a segmented drive system is, in most cases, mandatory.
The overall drive system may also incorporate a flexible coupling between the drive shaft segments for accommodating angular misalignment between the shaft ends. Such misalignment can be due, in part, to the vibratory loads acting on the drive shaft segments, as well as the overall flexibility of the supporting structure. In a helicopter aircraft, where vibratory loads can cause excessive motion of the shafts relative to one another, e.g., misalignment of up to 2.5 degrees, the drive system must provide substantial flexibility while maintaining torque transmission and a lightweight configuration. An example of such a flexible coupling is described in U.S. Pat. No. 4,457,734.
When a drive system is operating, it is necessary that continuous engagement be maintained between the drive shaft segments for transmitting power. However, when the system is inoperative, e.g., when power is not being transmitted from the power source to the driven component, continuous engagement of the drive shaft segments is no longer required and, in some instances, complete disengagement is preferred. An example where complete disengagement would be preferred is on a helicopter which has a folding tail section for minimizing storage space on an aircraft carrier or during transport in the cargo bay of a plane. On such an aircraft, the drive shaft segments must be disengaged from one another to permit the tail section to fold about a hinge.
In order for two drive shaft segments to be disengagable the coupling device must be substantially more complex then the system disclosed in U.S. Pat No. 4,457,734, yet must remain lightweight. In many instances, the engagement/disengagement must be accomplished relatively quickly, thus requiring the coupling device to be actuatable. The coupling device disclosed in U.S. Pat. No. 4,457,734 does not provide a means for quick disengagement.
One prior art method for addressing this concern is through the use of a spline arrangement wherein male splines, disposed on the end of one drive shaft segment, slidably mate with female splines of an adjacent drive shaft segment. The female splines are slidably disposed on the outer diameter of the drive shaft segment and urged into engagement by means of a coil spring. The torque is transmitted from the first drive shaft segment through the splines to the second drive shaft segment. Disengagement is accomplished by forcing the female spline to disengage from the male spline by compressing the coil spring.
A second type of coupling device that is currently used consists of two face gears disposed on the ends of the drive shafts to be coupled. The gear teeth of the face gears mesh with one another, transmitting load therebetween. The face gears are held in mesh by a coil spring which forces one face gear into the other. Disengagement of the face gears is accomplished by compressing the coil spring.
One difficulty associated with the disengagable coupling devices described above is that excessive flexing or motion of the drive shaft segments or their support structure could result in disengagement of the splines or gear teeth, resulting in loss of power transmission between the drive shaft segments.
The flexing of the drive shaft segments or its support structure can also result in excessive wear of the splines or gear teeth, thus requiring frequent maintenance and repair of the same.
Furthermore, an additional deficiency that exists with prior art coupling devices having intermeshing face gears, relates to the manner in which torque is transferred between the face gears. The input gear transmits the torque to the output gear as normal and tangential components on each tooth face. The tangential components of the torque serve to force the face gears apart. The constant forcing apart of the face gears results in significant wear of the individual gear teeth, necessitating continual monitoring and repair. Utilizing a stiffer coil spring would reduce this occurrence but would thereby require a considerably stronger means for compressing the coil spring when disengagement is desired. This results in a relatively heavy design and, therefore, is not a practical solution.
A need, therefore, exists for providing a coupling device which prevents disengagement of two drive shaft segments and minimizes axial displacement thereof, while providing a means for quick disengagement when desired. It is also desired that the coupling device provide a means for accommodating angular misalignment of the shaft segments.